Midblock sulfonated block copolymers are known. Typically, they are sulfonated polymers based on styrene and/or t-butyl styrene with the former predominantly used in a midblock, that is subsequently sulfonated and the latter in the endblocks, that resist sulfonation. These polymers are in a solid state in the presence of water and have both high water transport properties and sufficient wet strength. These polymers are known to have excellent barrier properties.
From WO2007010039 a midblock sulfonated styrenic block copolymer is known. This block copolymer is based on a block copolymer that comprises at least two polymer end blocks A and at least one polymer interior block B wherein each A block is a polymer block resistant to sulfonation and each B block is a polymer block susceptible to sulfonation, and wherein said A and B blocks do not contain any significant levels of olefinic unsaturation.
Such polymers are now commercially available for instance under the trademark Nexar® from Kraton Polymers. The typical structure of a Nexar molecule is a pentablock consisting of two poly(t-butylstyrene) (tBS) blocks, two poly(ethylene/propylene) (EP) blocks (hydrogenated polyisoprene), and in the middle a partly sulfonated polystyrene (sPS) block.
Such midblock sulfonated block copolymers are typically delivered to customers as a solution of about 10% in a combination of heptane and cyclohexane. For some customers this poses a problem because they are not used to handling this type of solvent and do not have adequate ventilation and disposal systems in place. Organic solvents may cause various handling problems due to the high volatility and low flame point of such solvents. The possibility to deliver such polymers as an aqueous emulsion would be a solution. Moreover, a waterborne system is more environmental friendly. Preparing a suitable aqueous emulsion, however, is not without its own problems.
EP2242137 and EP1852928 concern a membrane-electrode assembly for polymer electrolyte fuel cells. They employ a block copolymer comprising a polymer block (A) having ion-conductive groups and a polymer block (B) having no ion-conductive groups, both polymer blocks phase-separated from each other, polymer block (a) forms a continuous phase. In paragraph [0047] of EP2242137 it describes methods of emulsifying the block copolymer. This is described and illustrated for end-block sulfonated block copolymers only. End-block sulfonated block copolymers behave differently from the midblock sulfonated block copolymers. A method for preparing an aqueous dispersion of a midblock sulfonated block copolymer is therefore not disclosed in EP2242137 or EP1852928.
Unfortunately, midblock sulfonated block copolymers frequently require the presence of surfactants to prepare a stable emulsion. This adds in cost, complicates matters (foaming!) and may adversely affect the applicability of the aqueous emulsion.
Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant contains both a water insoluble (or oil soluble) component and a water soluble component. Surfactants will diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, in the case where water is mixed with oil. The water-insoluble hydrophobic group may extend out of the bulk water phase, into the air or into the oil phase, while the water-soluble head group remains in the water phase. This alignment of surfactants at the surface modifies the surface properties of water at the water/air or water/oil interface. (Wikipedia, 2014) To avoid any doubt, midblock sulfonated styrenic block copolymers are not considered surfactants. Rather, surfactants are compounds with a mass typically less than 5000 Dalton.
Ideally it should be possible to produce stable emulsions without surfactant, i.e., emulsions wherein the midblock sulfonated block copolymers remain finely dispersed as small particles, e.g. with an average particle size that is no greater than 2 micrometer, in water for a reasonable period of time. For commercial purposes this means a stable emulsion for at least 1 year. Under laboratory testing procedures this means that the emulsion is stable for at least 3 days without collapse or coalescence of the polymer particles. This should be possible, even when using a hydrocarbon solvent wherein the preceding polymerization has been performed. Expressed differently, when using a typical solution of a midblock sulfonated styrenic block copolymer dissolved in a hydrocarbon solvent such as cyclohexane, heptane or a mixture thereof, it would be ideal if a stable emulsion can be prepared without having to rely on an added surfactant.